Schottky diodes are well-known in GaAs and Si technologies. Some Si-based Schottky diodes have fairly low turn-on voltages but suffer from high series resistance and relatively poor frequency response. Si-based Schottky diodes are not easily integrated into InP-based semiconductor fabrication.
Low turn-on voltages are often desirable for Schottky diodes, particularly for mixer applications. U.S. Pat. No. 5,665,999 issued Sep. 9, 1997 to H. Brugger, teaches that low turn-on voltage Schottky diodes permit a low conversion loss without requiring a high pumping capacity in the local oscillator, and without using a bias voltage which may lead to Townsend current-hum disturbances. A GaAs Schottky device with reduced turn-on voltage is taught as fabricated using a graded In.sub.x Ga.sub.(1-x)As layer over a GaAs substrate, where the In content, x, increases continuously in the direction of the metal contact. Compatibility with InP is not taught, and the spatially varying lattice mismatch created by this process is thought potentially unreliable with InP technologies.
Schottky diodes compatible with InP semiconductor technology are known. For example, U.S. Pat. No. 5,652,435, issued Jul. 29, 1997 to E. Martin et al., describes a Schottky diode optical detector wherein the light-sensitive InGaAs layer is cladded front and back with InAlAs current-blocking layers. This device has a relatively high turn-on voltage similar to that of conventional GaAs Schottky diodes.
Some investigation into the relationship between Schottky barrier height and the proportion of In in In.sub.x Al.sub.1-x As material on an InP substrate has been undertaken by C. L. Lin, et al. in "Composition dependence of Au/In.sub.x Al.sub.1-x As Schottky barrier heights," Applied Physics Letters 49 (23), Dec. 8, 1986, pp. 1593-1595. Lin performs tests on the noted compound for 0.45.ltoreq.x.ltoreq.0.55, a range producing relatively high Schottky barriers.
InP-compatible Schottky diodes using a graded superlattice Schottky layer are also known. For example, U.S. Pat. No. 5,198,682 issued Mar. 30, 1993 to C.-S. Wu et al. describes an infrared photodetector using a superlattice having a graded dopant concentration, thereby developing an internal field which aids in the collection of photoexcited carriers. Lee et al., in Appl. Phys. Lett. Vol. 54, May 1989, pp. 1863-1865, teaches use of a graded superlattice of InGaAs/InAlAs to obtain enhanced Schottky barrier height U.S. Pat. No. 5,572,043, issued Nov. 5, 1996 to Shimizu et al. describes another SL-based Schottky diode, using sublayers of variously lattice-mismatched compounds and alternating compressively and tensilely strained layers to balance the mismatches. Each of these three references describe Schottky diodes having a relatively high turn-on voltage of .about.0.6V-0.7V.
InP-compatible Schottky diodes having high turn-on voltages are known. For example, U.S. Pat. No. 4,471,367 issued Sep. 11, 1984 to C. Chen et al. discloses a MESFET-gate Schottky structure composed of a thin, heavily doped InGaAs layer overlying a layer of low-doped InGaAs having an increased barrier height. Another example is U.S. Pat. No. 4,954,851, issued Sep. 4, 1990 to W. Chan, which describes a Schottky diode with a cadmiumcontaining layer overlying an InAlAs layer. The Cd-containing layer enhances the barrier height of the diode.
Thus, Schottky diodes with reduced turn-on voltages exist but are not known to be compatible with InP-based fabrication techniques. Schottky diodes compatible with InP are known, but all have conventional or high tun-on voltages. Yet, InP-based fabrication technologies are desirable, and low turn-on voltage Schottky diodes are particularly useful in certain applications, such as high-frequency mixers. A need therefore exists for low turn-on voltage Schottky diodes, and a method of making the same, which are compatible with InP fabrication technologies.